The present invention relates to a method for cutting a glass duct, such as a gas chromatography column, a glass fiber, and the like, of which the external surface is provided with a coating layer.
Such a method is known from U.S. Pat. No. 4,852,244. This document relates to the connection of optical fibers whereby an insulating layer must be removed from the fiber over a predetermined length, whereafter a transverse score is formed in the bared fiber portion and the said fiber portion is broken.
Dutch patent application 1004496 discloses a gas chromatograph which comprises an oven with an injector and a detector, which injector and detector can be connected to each other by a capillary gas chromatography column. This gas chromatography column comprises a capillary tube of glass having an inside diameter in the order of 50 to 600 xcexcm, which tube is provided, on the outside, with a coating, in particular a polyamide coating layer, which reduces the fragility of the tube. Such gas chromatography columns are to be replaced regularly, for instance because for measurement on one substance, a different column is used than for measurement on another substance. In the gas chromatograph such as it is described in the above Dutch patent application, to that end, use is made of coupling devices with a constriction, on one side of which a glass duct having preferably the same inside and outside diameters as those of a capillary gas chromatography column and having a fixed standardized length is fixedly connected, which glass duct reaches to the proper depth into the injector chamber of the injector and/or the detection space of the detector. To the other side of this coupling device, the removable capillary gas chromatography column can be connected. Here, it is of great importance that the gas chromatography column, when it is inserted into the constriction mentioned, is cut off completely straight. A gas chromatography column which has not been cut off straight disturbs the measuring results.
At the time of delivery, a gas chromatography column to be installed is typically protected at the end by a so-called septum, which is a rubber or plastic cap into which the end of the column is inserted. Upon removal of this cap, the end of the column can become contaminated, and particles of this septum may end up, via the column and the gas flowing through it, in the oven and be combusted there, thereby disturbing the measuring values. Although this adverse effect can be prevented by making the septum from glass, a rubber or plastic septum is still frequently used in practice. A septum-contaminated end of a gas chromatography column to be installed should be cut off. Here, too, it holds that a gas chromatography column which has not been cut off straight disturbs the measuring results.
The end of the glass duct or gas chromatography column to be inserted into the injector or into the detector can be secured therein by a clamped joint, in that a hardened steel nut, in particular a stainless steel nut, and a cone-shaped ferrule are fitted over the glass duct or gas chromatography column, whereafter the glass duct or the gas chromatography column is slid into a hardened steel cone-shaped assembly part, in particular a stainless steel assembly part, and by tightening the nut, the ferrule is pressed into the cone-shaped assembly part. The ferrule is typically made of carbon or vespel, which is a specific polymer which is capable of resisting high temperatures without releasing molecular contaminants and deforms upon tightening of the nut, thereby yielding a gas-tight seal and securement with respect to the injector or the detector. Even so, as the ferrule is fitted over the end of the glass duct or the gas chromatography column, particles of the ferrule may still contaminate the end of the glass duct or the gas chromatography column. This possibly contaminated end must therefore be cut off. In this case, too, when the cut end is inserted into the injector or the detector and the end has not been cut off completely straight, this can cause errors in the measuring results.
In cutting off the gas chromatography column or the hollow glass duct, account is to be taken of the fact that the length of the column and the glass duct, as well as the depth over which they are inserted into the injector and the detector, must have a defined value, typically a standardized value, to obtain mutually comparable measuring results.
Not only in gas chromatography columns, or similar hollow glass ducts, is it of importance that they can be cut off straight, but also in glass fibers, for instance for telecommunication purposes, this is of great importance. Such glass fibers, herein understood to be encompassed by the term xe2x80x98glass ductxe2x80x99, are formed by a solid glass core having around it a vapor-deposited glass layer of a different composition having a different refractive index, and here too, a coating, in particular a polyamide coating layer, is provided around the whole for the glass fiber to retain its strength. To provide that the end of such a glass fiber retains its optical properties undisturbed, it is of importance, here too, for this end to be cut off completely straight.
When a glass duct, in particular a gas chromatography column or a glass fiber, which is provided with a coating layer, is cut off in the manner as is known in ordinary glass cutting, the glass duct is notched and then broken off. The coating layer is thereby torn up on one side and the glass duct exhibits a frayed end, while, further, particles are released which cause contamination, especially in a gas chromatography column. Due to the presence of the coating layer which is partly torn, breaking off the glass duct is a non-controlled process, which makes it uncertain whether the glass duct has been cut off straight, nor is it known to what extent it has not been cut off straight. The result is a glass duct having non-reproducible properties, which introduces measuring errors or optical deviations, depending on the application of the glass duct.
The object of the invention is to eliminate these disadvantages, at least to limit them to a considerable extent, and to provide a method for cutting a glass duct, whereby this glass duct can be cut off completely straight, as well as a device to enable the straight cutting operation.
To that end, the method such as it is described in the preamble is characterized, according to the invention, in that before the notching and subsequent breaking off of the glass duct, first the coating layer is cut through all round at the axial location of the fiber where the fiber is to be notched. To enable the use of this method for glass ducts of varying diameters, the glass duct can be retained in a fixed position during cutting, while a cutting member, as well as a counterpressure surface for the cutting member, are moved all round the glass duct. Specifically the counterpressure surface is of importance here, because glass ducts having a relatively small diameter are pressed further sidewards by the cutting member than are glass ducts having a relatively great diameter, and therefore this sideway displacement should be effectively limited to prevent the glass ducts from breaking in a manner not intended.
In a more concrete method, a glass duct is inserted in an insertion passage through a housing, the glass duct being retained in a first part of the housing by clamping means, and during the cutting of at least the coating layer, a second part of the housing, which includes the cutting member and, if present, the counterpressure surface, is moved around the glass duct. In the condition of rest, the insertion passage through the housing is at least partly closed off by the cutting member, while to clear the insertion passage, the cutting member is moved against the action of a compression spring, such that a glass duct can be led through the insertion passage, whereafter the cutting member, under the action of the compression spring, presses against the inserted glass duct and, when moved around this glass duct, cuts through at least the coating layer.
To obtain a completely straight, or at least substantially completely straight cut, the device for cutting a glass duct, such as a gas chromatography column, a glass fiber, and the like, of which the external surface is provided with a coating layer, is characterized, according to the invention, in that the device comprises a housing through which a glass duct can be inserted, which housing comprises a first part with an insertion passage, which first part is provided with clamping means for clamping a glass duct inserted through the housing, and a second part with an insertion passage being aligned with the insertion passage through the first part, which second part can rotate relative to the first part substantially about the axis of the two aligned insertion passages, which second part includes a cutting member which, under spring action, presses perpendicularly against a glass duct inserted through the housing.
In a preferred embodiment, in the insertion passage of the second part of the housing, a guide bush is arranged, one end of which extends as far as, or substantially as far as, the cutting member and forms a counterpressure surface for the cutting member. In particular, this guide bush is cut slantwise, with the portion of the guide bush that continues farthest being located opposite, or substantially opposite, the cutting member.